Since the rampage, the FBI has waged a high-profile fight with Apple over accessing a phone used by one of the terrorists, pleaded with the public for information and charged people related to the shooters with other crimes unearthed during the investigation.

But a year after one of the deadliest attacks in the United States since Sept. 11, 2001, the FBI is still trying to determine whether anyone helped the attackers and hoping to figure out what the couple did during a mysterious period of time before dying in a shootout with police.

On the morning of Dec. 2, 2015, workers with San Bernardino County’s health department were gathered for a training session when Syed Rizwan Farook, one of the employees, left and returned with his wife, Tashfeen Malik. The pair began shooting at Farook’s co-workers, ultimately firing more than 100 rounds before fleeing, according to a report released by the Justice Department. Four hours later, police tracked the couple down and killed them during a frenzied gun battle on a residential street.

The FBI said it was investigating the shooting as an act of terrorism after it emerged that Malik had posted a pledge of allegiance on Facebook to the leader of the Islamic State, a message made on behalf of both attackers. Authorities say both were radicalized for some time before the attack, although investigators said they do not think Malik and Farook were directly guided by a foreign terrorist group.

“The brutality of the attack’s perpetrators could not have been in starker contrast to the selflessness and generosity that characterized those taken from us,” the White House said in a statement marking the anniversary Friday and honoring the victims of the attack. Using another acronym for the Islamic State, the message continued: “In the year since this tragedy, we have mourned those we lost, just as we have continued to confront the violent ideology behind this attack as well as the terrorist groups, including ISIL, that propagate it.”

In a new interview broadcast this week, the police chief in San Bernardino said authorities believe that the specifics of the attack — targeting that particular gathering at that time — may have been motivated by the holiday party set to take place in the same room after the training ended. The chief, Jarrod Burguan, cited comments made by the female attacker before the shooting.

Malik had said online “that she didn’t think that a Muslim should have to participate in a non-Muslim holiday or event,” Burguan told ABC News.

The room where the training occurred — the same room where the health department had held active-shooter training earlier that year — was filled at the time with Christmas decorations, including a large Christmas tree, ornaments and items on the walls.

Through traffic cameras, surveillance footage, witnesses and other resources, investigators have figured out some of what Farook and Malik did during the four hours. About 45 minutes after the shooting, they visited the city’s Lake Seccombe, so evidence recovery divers were dispatched into the water to see what they could find. None of the items recovered appeared to be relevant to the investigation, the FBI said.

There is still a window between 12:59 p.m. and 1:17 p.m. where, as Bowdich put it earlier this year, “we’re dark.”

The iPhone fight

The investigation has not resulted in any charges directly stemming from the attack, but authorities are still looking at “whether Farook and Malik were supported in any way in the planning, financing or execution of the attack,” Laura Eimiller, an FBI spokeswoman, said in an email.

FBI Director James B. Comey had argued that it was necessary for Apple to help unlock the phone. (Jonathan Ernst/Reuters)

Apple argued that the fight was about “much more than a single phone or a single investigation” and could threaten civil liberties, while Comey, the FBI director, said investigators needed to access the phone because it was possible it “holds the clue to finding more terrorists.”

While investigators continue checking whether anyone else knew about the attack, a particularly high-profile voice has repeatedly suggested that there are people who knew Farook and Malik had amassed explosives and guns and never reported it.

President-elect Donald Trump has asserted that numerous people saw these devices in the couple’s home and never said anything. (He has used this claim to argue that American Muslims need to report suspicious activity, which the FBI says they already do.)

“In San Bernardino, many people saw the bombs all over the apartment of the two people that killed 14 and wounded many, many people,” Trump said during the second presidential debate, adding: “Muslims have to report the problems when they see them.”

One person working in the neighborhood told a local outlet he had seen numerous Middle Eastern men in the area but did not report anything to avoid racial profiling. Another man told a local broadcast station that an unspecified person saw purported suspicious activity — including the couple getting numerous deliveries and working in the garage at odd times — but, again, did not want to be seen as involved in racial profiling.

Neither of these accounts included anyone saying they “saw bombs all over the apartment” of the attackers, as Trump asserted. Eimiller, the FBI spokeswoman, said investigators have not substantiated any of those accounts given to news outlets.

A makeshift vigil near the shooting site last year. (Jabin Botsford/The Washington Post)

“What kind of parent makes plans to abandon their child?” she told The Washington Post recently. “How were they capable of something like that and we didn’t know?”

Federal authorities have said that Farook had also plotted at least two other attacks years before he spoke with Malik, making detailed plans with his friend, Enrique Marquez Jr., who was arrested last year.

In a courtroom sketch, Enrique Marquez Jr. appeared in federal court. (Bill Robles/AP)

Authorities said there is no evidence that Marquez had any involvement in the attack last year, but they charged him with buying the guns that would be used in the shooting and said he bought explosive material found in a pipe bomb left behind by Farook and Malik.

Marquez is one of four people connected to the attackers who has been arrested and charged since the shooting, although none were accused of any direct involvement.

Earlier this year, officials arrested Farook’s brother, Syed Raheel Farook, as well as Raheel’s wife, Tatiana Farook, and her sister, Mariya Chernykh. Marquez and Chernykh are married, and they are all accused of committing fraud or lying to keep up a sham marriage so that she could get immigration benefits.

Raheel, his wife and his sister-in-law are all set to stand trial in March, while Marquez’s trial is set for next September, nearly two years after he was arrested.

This story, first published at 10:41 a.m., has been updated with the White House’s statement.

From Columbine to San Bernardino, here's a look at some of the notable U.S. mass shootings since 1999. (Alice Li/The Washington Post)

Mark Berman covers national news for The Washington Post. He also anchors Post Nation, The Post's hub for news, analysis and the biggest stories happening across the country. He has been at The Post since 2007 and previously covered transportation and local news.

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